The present invention relates to an adhesive made from a polymodal asymmetric elastomeric block copolymer, more particularly, to such an adhesive for forming high strength bonds to low surface energy surfaces and, even more particularly, to articles made with such adhesives, including adhesive tapes.
Block copolymers are known in the art for a variety of applications including for use in removable tape applications wherein the tape is typically removed when it is no longer needed. See, for example, U.S. Pat. Nos. 5,393,787 and 5,296,547, both of which are assigned to the present assignee. Such block copolymers can be formulated into a pressure sensitive adhesive, which may be used to make a variety of different types of tapes including removeable tapes. Specific examples of the various tapes which may be made include masking tapes, packaging tapes, medical tapes and autoclave indicator tapes. Additionally, the pressure sensitive adhesive may be used to make protective sheeting, labels, and facestock.
Articles incorporating a polymer foam core are known. The foam includes a polymer matrix and is characterized by a density that is lower than the density of the polymer matrix itself. Density reduction is achieved in a number of ways, including through creation of gas-filled voids in the matrix (e.g., by means of a blowing agent) or inclusion of polymeric microspheres (e.g., expandable microspheres) or non-polymeric microspheres (e.g., glass microspheres).
The adhesives of the present invention are particularly useful for forming strong bonds to low surface energy substrates. As used herein, low surface energy substrates are those having a surface energy of less than about 45 dynes per centimeter, more typically less than about 40 dynes per centimeter, and most typically less than about 35 dynes per centimeter.
In one aspect of the invention, a pressure sensitive adhesive is provided that comprises 100 parts by weight of a polymodal asymmetric elastomeric block copolymer and at least one tackifier or tackifying resin in an amount sufficient to raise the calculated Fox Tg of the rubber phase of the adhesive to greater than 245xc2x0 K. The amount of tackifier used depends on the resulting Tg, of the adhesive""s rubber phase, that is obtained by the addition of the tackifier. So, more tackifier can be added to obtain the Tg desired. The adhesive composition may also include 0 to about 50 parts by weight of a crosslinking agent and 0 to about 300 parts by weight of a plasticizer. In general, the difference between a tackifier and a plasticizer is that the addition of a tackifier increases the Tg of the adhesive""s rubber phase while the addition of the plasticizer decreases the Tg of the adhesive""s rubber phase. The polymodal asymmetric elastomeric block copolymer has the formula QnY and comprises from about 4 to about 40 percent by weight of a polymerized monovinyl aromatic compound and from about 96 to about 60 percent by weight of polymerized conjugated diene. Q represents an individual arm of the block copolymer and has the formula S-B; n represents the number of arms Q in the block copolymer and is a whole number of at least 3; and Y is the residue of a multifunctional coupling agent. S is a nonelastomeric polymer segment endblock of a polymerized monovinyl aromatic homopolymer, there being at least two different molecular weight endblocks in the copolymer, a higher molecular weight endblock and a lower molecular weight endblock. The number average molecular weight of the higher molecular weight endblock (Mn)H is in the range of from about 5,000 to about 50,000. The number average molecular weight of the lower molecular weight endblock (Mn)L is in the range of from about 1,000 to about 10,000. The ratio (Mn)H/(Mn)L is at least 1.25. B is an elastomeric polymer segment midblock which connects each arm to the residue of a multifunctional coupling agent (Y) and comprises a polymerized conjugated diene or combination of conjugated dienes.
The adhesive has a rubber phase exhibiting a calculated Fox Tg of greater than 245xc2x0 K, and the adhesive forms a high strength bond to low surface energy surfaces. As used herein, low surface energy surfaces or substrates exhibit a surface energy of less than 45 dyne/cm, more typically less than 40 dyne/cm, or more typically less than 35 dyne/cm. Preferably, the rubber phase of the adhesive has a calculated Fox Tg of at least 250xc2x0 K and, more preferably, 255xc2x0 K. In addition, the rubber phase of the adhesive preferably has a calculated Fox Tg with an upper limit of less than 300xc2x0 K and, more preferably, an upper limit of 285xc2x0 K. In general, the ability of the present inventive adhesive to bond to low surface energy surfaces increases as the Tg of the rubber phase increases. The Tg of the rubber phase is dependent on the weight fraction (i.e., concentration) and the Tg of each of the various components of the adhesive, as well as the weight fraction and Tg of the rubber portion of the copolymer.
The present inventive adhesive can exhibit a 180xc2x0 peel strength on a low surface energy substrate (e.g., high density polyethylene, or HDPE) of at least about 20 Newtons per decimeter (N/dm), preferably, at least about 60 N/dm, more preferably at least about 80 N/dm, and most preferably at least about 100 N/dm, for example, when the adhesive has a thickness of about 5 mil (125 xcexcm) and is, for example, in the form of a film (e.g., a transfer tape).
The present adhesive can be used in combination with a backing (e.g., a foam core, a vinyl strip or sheet, etc.) having first and second major surfaces, with the adhesive coated on at least a portion of at least one of the major surfaces. The backing can include a release surface (e.g., for a tape roll). The backing can also be a foam tape core made of the same or a different polymodal asymmetric elastomeric block copolymer, and the adhesive can be in the form of at least one co-extruded layer on the foam tape core. The backing can also be an acrylic foam tape core and the adhesive in the form of at least one co-extruded layer on the foam tape core. The backing can be in the form of a foam, with at least one of its major surfaces being substantially smooth, having an Ra value less than about 75 micrometers, as measured by laser triangulation profilometry, and comprising a plurality of microspheres, at least one and preferably a plurality of which are expandable polymeric microspheres. Typically, this foam is substantially free of broken polymeric microspheres. The present adhesive can also be used in combination with at least one other polymer composition in the form of a plurality of discrete structures bonded to or embedded in the foam.
The present adhesive can exhibit a 90xc2x0 peel strength on a low surface energy substrate (e.g., HDPE) of at least about 50 N/dm, preferably, at least about 75 N/dm, more preferably at least about 100 N/dm and most preferably at least about 150 N/dm, for example, when the adhesive has a thickness of about 3 mil (75 xcexcm) to about 5 mil (125 xcexcm) and is, for example, in the form of an adhesive skin laminated onto, or co-extruded with, an adhesive or non-adhesive foam tape core having a thickness of about 1 mm. In general, the thicker the adhesive, with or without a foam core, the higher the bond strength exhibited by the adhesive, up to a limit, such as the cohesive strength of the foam.
Preferably, the tackifier used in the present adhesive is a low acidic or neutral tackifier. As used herein, a low acidic or neutral tackifier is one with an acid number of about 1 mg KOH/g or less, as tested according to Exxon Chemical Co. analytical method specification AMS 360.25. In addition, the tackifier preferably has a Tg, as measured by differential scanning calorimeter (DSC), in the range of from about xe2x88x9250xc2x0 C. to about 200xc2x0 C. and, more preferably, from about xe2x88x9230xc2x0 C. to about 150xc2x0 C. The Tg of the adhesive""s rubber phase is dependent, in significant part, on the Tg of the tackifier. In general, for a given weight of tackifier, as the Tg of the tackifier increases, the Tg of the adhesive increases. It is also preferable for the tackifier to have a softening point of above 80xc2x0 C., and more preferably of 90xc2x0 C. or higher. Preferably, the present adhesive comprises at least one tackifier selected from the group consisting of hydrogenated mixed aromatic tackifiers, aliphatic/aromatic hydrocarbon liquid tackifiers; naphthenic oils, mineral oils, and a mixture of one or more thereof. It can be desirable for the adhesive to comprise in the range of from about 50 parts to about 350 parts, preferably, from about 70 parts to about 300 parts and, more preferably, from about 90 parts to about 265 parts by weight of one or more tackifiers.
The present adhesive can be a radiation crosslinkable composition such as, for example, by electron beam radiation, ultraviolet radiation, etc., so as to produce a crosslinked polymodal asymmetric elastomeric block copolymer.
In an aspect of the invention, an article is provided that includes a polymer foam having a substantially smooth surface. The foam may be provided in a variety of shapes, including a rod, a cylinder, a sheet, etc. In some embodiments, e.g., where the foam is provided in the form of a sheet, the foam has a pair of major surfaces, one or both of which are substantially smooth. The foam includes a plurality of microspheres, at least one of which is an expandable polymeric microsphere.
As used herein, a xe2x80x9cpolymer foamxe2x80x9d refers to an article that includes a polymer matrix in which the density of the article is less than the density of the polymer matrix alone.
A xe2x80x9csubstantially smoothxe2x80x9d surface refers to a surface having an Ra value less than about 75 micrometers, as measured by laser triangulation profilometry according to the procedure described in the Examples, infra. Preferably, the surface has an Ra value less than about 50 micrometers, more preferably less than about 25 micrometers. The surface is also characterized by the substantial absence of visually observable macroscopic defects such as wrinkles, corrugations and creases. In addition, in the case of an adhesive surface, the surface is sufficiently smooth such that it exhibits adequate contact and, thereby, adhesion to a substrate of interest. The desired threshold level of adhesion will depend on the particular application for which the article is being used.
An xe2x80x9cexpandable polymeric microspherexe2x80x9d is a microsphere that includes a polymer shell and a core material in the form of a gas, liquid, or combination thereof, that expands upon heating. Expansion of the core material, in turn, causes the shell to expand, at least at the heating temperature. An expandable microsphere is one where the shell can be initially expanded or further expanded without breaking. Some microspheres may have polymer shells that only allow the core material to expand at or near the heating temperature.
The article is a pressure sensitive adhesive article when the article has a surface available for bonding that is either tacky at room temperature (i.e., pressure sensitive adhesive articles) or becomes tacky after being heated (i.e., heat-activated adhesive articles). An example of an adhesive article is a foam that itself is an adhesive, or an article that includes one or more separate adhesive compositions bonded to the foam, e.g., in the form of a continuous layer or discrete structures (e.g., stripes, rods, filament, etc.), in which case the foam itself need not be an adhesive. Examples of non-adhesive articles include non-adhesive foams and adhesive foams provided with a non-adhesive composition, e.g., in the form of a layer, substrate, etc., on all surfaces available for bonding.
The foam can be substantially free of urethane crosslinks and urea crosslinks, thus eliminating the need for isocyanates in the composition. An example of such a material for the polymer foam is an acrylic polymer or copolymer. In some cases, e.g., where high cohesive strength and/or high modulus is needed, the foam may be crosslinked.
The polymer foam preferably includes a plurality of expandable polymeric microspheres. The foam may also include one or more non-expandable microspheres, which may be polymeric or non-polymeric microspheres (e.g., glass microspheres).
Examples of preferred expandable polymeric microspheres include those in which the shell is essentially free of vinylidene chloride units. Preferred core materials are materials other than air that expand upon heating.
The foam may contain agents in addition to microspheres, the choice of which is dictated by the properties needed for the intended application of the article. Examples of suitable agents include those selected from the group consisting of tackifiers, plasticizers, pigments, dyes, solid fillers, and combinations thereof. The foam may also include gas-filled voids in the polymer matrix. Such voids typically are formed by including a blowing agent in the polymer matrix material and then activating the blowing agent, e.g., by exposing the polymer matrix material to heat or radiation.
The properties of the article may be adjusted by bonding and/or co-extruding one or more polymer compositions (e.g., in the form of continuous layers or discrete structures such as stripes, rods, filament, etc.) to or into the foam. Both foamed and non-foamed compositions may be used. A composition may be bonded directly to the foam or indirectly, e.g., through a separate adhesive.
The article may be used as a xe2x80x9cfoam-in-placexe2x80x9d article. The term foam-in-place refers to the ability of the article to be expanded or further expanded after the article has been placed at a desired location. Such articles are sized and placed in a recessed area or on an open surface, and then exposed to heat energy (e.g., infrared, ultrasound, microwave, resistive, induction, convection, etc.) to activate, or further activate, the expandable microspheres or blowing agent. Such recessed areas can include a space between two or more surfaces (e.g., parallel or non-parallel surfaces) such as found, for example, between two or more opposing and spaced apart substrates, a through hole or a cavity. Such open surfaces can include a flat or uneven surface on which it is desirable for the article to expand after being applied to the surface. Upon activation, the foam expands due to the expansion of the microspheres and/or blowing agent, thereby partially or completely filling the recess or space, or thereby increasing the volume (e.g. height) of the article above the open surface.
It can be desirable for the foam to comprise a substantially uncrosslinked or thermoplastic polymeric matrix material. It can also be desirable for the matrix polymer of the foam to exhibit some degree of crosslinking. Any crosslinking should not significantly inhibit or prevent the foam from expanding to the degree desired. One potential advantage to such crosslinking is that the foam will likely exhibit improved mechanical properties (e.g., increased cohesive strength) compared to the same foam with less or no crosslinking. In the case of foams having a curable polymer matrix, exposure to heat can also initiate cure of the matrix.
It can further be desirable for the foam-in-place article to comprise multiple layers, discrete structures or a combination thereof (See, for example, FIGS. 4-6 and the below discussion thereof), with each layer and discrete structure having a difference in the way it foams-in-place (e.g., using expandable microspheres, blowing agents or a combination thereof), a difference in the degree to which it can be expanded in place, or a combination thereof. For example, the concentration of expandable microspheres and/or blowing agents can be different, the type of expandable microspheres and/or blowing agents can be different, or a combination thereof can be used. In addition, for example, one or more of the layers and discrete structures can be expandable in place while one or more other layers and discrete structures can be unexpandable in place.
In yet another aspect of the invention, an article (e.g., an adhesive article, as defined above) is provided that comprises a polymer foam (as defined above) that includes: (a) a plurality of microspheres, at least one of which is an expandable polymeric microsphere (as defined above), and (b) a polymer matrix that is substantially free of urethane crosslinks and urea crosslinks. The matrix can include a blend of two or more polymers in which at least one of the polymers in the blend is a pressure sensitive adhesive polymer (i.e., a polymer that is inherently pressure sensitive, as opposed to a polymer which must be combined with a tackifier in order to form a pressure sensitive composition) and at least one of the polymers is selected from the group consisting of unsaturated thermoplastic elastomers, acrylate-insoluble saturated thermoplastic elastomers, and non-pressure sensitive adhesive thermoplastic polymers.
The foam preferably has a substantially smooth surface (as defined above). In some embodiments, the foam has a pair of major surfaces, one or both of which may be substantially smooth. The foam itself may be an adhesive. The article may also include one or more separate adhesive compositions bonded to the foam, e.g., in the form of a layer. If desired, the foam may be crosslinked.
The polymer foam preferably includes a plurality of expandable polymeric microspheres. It may also include non-expandable microspheres, which may be polymeric or non-polymeric microspheres (e.g., glass microspheres). The properties of the article may be adjusted by directly or indirectly bonding one or more foamed or non-foamed polymer compositions to the foam.
The invention also features multi-layer articles that include the above-described foam articles provided on a major surface of a first substrate, or sandwiched between a pair of substrates. Examples of suitable substrates include wood substrates, synthetic polymer substrates, and metal substrates (e.g., metal foils).
In yet a further aspect of the invention, a method is provided for preparing an article, where the method includes: (a) melt mixing a polymer composition and a plurality of microspheres, one or more of which is an expandable polymeric microsphere (as defined above), under process conditions, including temperature, pressure and shear rate, selected to form an expandable extrudable composition; (b) extruding the composition through a die to form a polymer foam (as defined above); and (c) at least partially expanding one or more expandable polymeric microspheres before the polymer composition exits the die. It can be preferable for most, if not all, of the expandable microspheres to be at least partially expanded before the polymer composition exits the die. By causing expansion of the expandable polymeric microspheres before the composition exits the die, the resulting extruded foam can be produced to within tighter tolerances, as described below in the Detailed Description.
It is desirable for the polymer composition to be substantially solvent-free. That is, it is preferred that the polymer composition contain less than 20 wt. % solvent, more preferably, contain substantially none to no greater than about 10 wt. % solvent and, even more preferably, contain no greater than about 5 wt. % solvent.
In an additional aspect of the invention, another method is provided for preparing an article that includes: (a) melt mixing a polymer composition and a plurality of microspheres, one or more of which is an expandable polymeric microsphere (as defined above), under process conditions, including temperature, pressure and shear rate, selected to form an expandable extrudable composition; and (b) extruding the composition through a die to form a polymer foam (as defined above). After the polymer foam exits the die, enough of the expandable polymeric microspheres in the foam remain unexpanded or, at most, partially expanded to enable the polymer foam to be used in a foam-in-place application. That is, the extruded foam can still be further expanded to a substantial degree at some later time in the application. Preferably, the expandable microspheres in the extruded foam retain most, if not all, of their expandability.
In another aspect of the invention, another method is provided for preparing an article that includes: (a) melt mixing a polymer composition and a plurality of microspheres, one or more of which is an expandable polymeric microsphere (as defined above), under process conditions, including temperature, pressure and shear rate, selected to form an expandable extrudable composition; and (b) extruding the composition through a die to form a polymer foam (as defined above) having a substantially smooth surface (as defined above). It is also possible to prepare foams having a pair of major surfaces in which one or both major surfaces are substantially smooth.
Polymers used according to the present invention can preferably possess a weight average molecular weight of at least about 10,000 g/mol, and more preferably at least about 50,000 g/mol. It can also be preferable for the polymers used according to the present invention to exhibit shear viscosities measured at a temperature of 175xc2x0 C. and a shear rate of 100 secxe2x88x921, of at least about 30 Pascal-seconds (Pa-s), more preferably at least about 100 Pa-s and even more preferably at least about 200 Pa-s.
The article may be an adhesive article (as defined above), e.g., a pressure sensitive adhesive article or a heat-activated adhesive article. In some embodiments, the foam itself is an adhesive.
Both the expandable extrudable composition and the extruded foam preferably include a plurality of expandable polymeric microspheres (as defined above). The extruded foam and the expandable extrudable composition may also include one or more non-expandable microspheres, which may be polymeric or non-polymeric microspheres (e.g., glass microspheres).
The expandable extrudable composition may be co-extruded with one or more additional extrudable polymer compositions, e.g., to form a polymer layer on a surface of the resulting foam. For example, the additional extrudable polymer composition may be an adhesive composition. Other suitable additional extrudable polymer compositions include additional microsphere-containing compositions.
The method may also include crosslinking the foam. For example, the foam may be exposed to thermal, actinic, or ionizing radiation or combinations thereof subsequent to extrusion to crosslink the foam. Crosslinking may also be accomplished by using chemical crosslinking methods based on ionic interactions.
The invention provides foam-containing articles, and a process for preparing such articles, in which the articles can be designed to exhibit a wide range of properties depending upon the ultimate application for which the article is intended. For example, the foam core may be produced alone or in combination with one or more polymer compositions, e.g., in the form of layers to form multi-layer articles. The ability to combine the foam with additional polymer compositions offers significant design flexibility, as a variety of different polymer compositions may be used, including adhesive compositions, additional foam compositions, removable compositions, layers having different mechanical properties, etc. In addition, through careful control of the foaming operation it is possible to produce a foam having a pattern of regions having different densities.
Both thin and thick foams can be produced. In addition, both adhesive and non-adhesive foams can be produced. In the latter case, the foam may be combined with one or more separate adhesive compositions to form an adhesive article. In addition, it is possible to prepare foams from a number of different polymer matrices, including polymer matrices that are incompatible with foam preparation processes that rely on actinic radiation-induced polymerization of microsphere-containing photopolymerizable compositions. Examples of such polymer matrix compositions include unsaturated thermoplastic elastomers and acrylate-insoluble saturated thermoplastic elastomers. Similarly, it is possible to include additives such as ultraviolet-absorbing pigments (e.g., black pigments), dyes, and tackifiers that could not be used effectively in actinic radiation-based foam processes. It is further possible, in contrast to solvent-based and actinic radiation-based foam processes, to prepare foams having a substantially homogeneous distribution of microspheres. In addition, the present expanded foam (i.e., a foam containing microspheres that have been at least partially expanded) can have a uniform size distribution of the expanded microspheres from the surface to the center of the foam. That is, there is no gradient of expanded microsphere sizes from the surface to the center of the foam, e.g., like that found in expanded foams which are made in a press or a mold. Expanded foams that exhibit such a size distribution gradient of their expanded microspheres can exhibit weaker mechanical properties than foams that have a uniform size distribution of the expanded microspheres. Oven foaming of these foam compositions requires long residence times in the high temperature oven due to the poor thermal conductivity of the foams. Long residence times at high temperatures can lead to polymer and carrier (e.g., release liner) degradation. In addition, poor heat transfer can also lead to foams containing non-uniform expansion, causing a density gradient. Such a density gradient can significantly decrease the strength and otherwise detrimentally impact the properties of the foam. The process associated with oven foaming is also complicated and usually requires unique process equipment to eliminate large scale corrugation and buckling of the planar sheet. For a reference on oven foaming see, for example, Handbook of Polymeric Foams and Foam Technology, eds: D. Klempner and K. C. Frisch, Hanser Publishers, New York, N.Y., 1991.
Foams with a substantially smooth surface can be produced in a single step. Accordingly, it is not necessary to bond additional layers to the foam in order to achieve a smooth-surfaced article. Substantially smooth-surfaced foams are desirable for a number of reasons. For example, when the foam is laminated to another substrate, the substantially smooth surface minimizes air entrapment between the foam and the substrate. Moreover, in the case of adhesive foams the substantially smooth surface maximizes contact with a substrate to which the foam is applied, leading to good adhesion.
The extrusion process enables the preparation of multi-layer articles, or articles with discrete structures, in a single step. In addition, when foaming occurs during the extrusion, it is possible, if desired, to eliminate separate post-production foaming processes. Moreover, by manipulating the design of the extrusion die (i.e., the shape of the die opening), it is possible to produce foams having a variety of shapes.
In addition, the present method may include heating the article after extrusion to cause further expansion. The additional expansion may be due to microsphere expansion, activation of a blowing agent, or a combination thereof.
It is also possible to prepare xe2x80x9cfoam-in-placexe2x80x9d articles by controlling the process temperature during the initial foam preparation such that expansion of the microspheres is minimized or suppressed. The article can then be placed at a location of use or application, (e.g., in a recessed area or on an open surface) and heated, or exposed to an elevated temperature to cause microsphere expansion. xe2x80x9cFoam-in-placexe2x80x9d articles can also be prepared by including a blowing agent in the expandable extrudable composition and conducting the extrusion process under conditions insufficient to activate the blowing agent. Subsequent to foam preparation, the blowing agent can be activated to cause additional foaming.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.